Hi,
From "Site administration > Reports > Security checks", i see a "critical" warning for "Default role for all users" with detail of "The default user role "Authenticated user" is incorrectly defined!".
But when i check, it's well the good role that is defined! And i've tried to reset it (and other default roles) to default permissions with no positive change 
I can see this problem on my main Moodle plateform (3.7.6), on another one (3.8.2), and even on Moodle 3.9 QA, and other people see the same thing.
So i think it's a bug in this check! I've seen MDL-50613 (closed), which means this should not indicate "critical" warning.
What do others think?
Séverin
PS : for french reading people, you can look at this french discussion about this problem.